Magneto-resistive memory cell structures with improved selectivity

ABSTRACT

A magneto-resistive memory comprises magneto-resistive memory cells comprising two pinned magnetic layers on one side of a free magnetic layer. The pinned magnetic layers are formed with anti-parallel magnetization orientations such that a net magnetic moment of the two layers is substantially zero. The influence of pinned magnetic layers on free magnetic layer magnetization orientations is substantially eliminated, allowing for increased predictability in switching behavior and increased write selectivity of memory cells.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/068,465, filed Feb. 6, 2002 (now U.S. Pat. No. 6,735,112).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magneto-resistive memories, and moreparticularly, to magneto-resistive memory cell structures that offersuperior selectivity of memory cells.

2. Description of the Related Art

As computer memory technology advances, magneto-resistive memory hasemerged as a possible replacement for conventional memory devices.Magneto-resistive memories are non-volatile and employ themagneto-resistive effect to store memory states. These memoriestypically use the magnetization orientation of a layer ofmagneto-resistive material to represent and to store a binary state. Forexample, magnetization orientation in one direction may be defined as alogic “0”, while magnetization orientation in another direction may bedefined as a logic “1”.

The ability to read stored binary states is a consequence of themagneto-resistive effect. This effect is characterized by a change inresistance of multiple layers of magneto-resistive material, dependingon the relative magnetization orientations of the layers. Thus, amagneto-resistive memory cell typically has two magnetic layers that maychange orientation with respect to one another. Where the directions ofthe magnetization vectors point in the same direction, the layers aresaid to be in a parallel orientation and where the magnetization vectorspoint in opposite directions, the layers are said to be orientedanti-parallel. In practice, typically one layer, the free or “soft”magnetic layer, is allowed to change orientation, while the other layer,the pinned or “hard” magnetic layer, has a fixed magnetizationorientation to provide a reference for the orientation of the freemagnetic layer. The magnetization orientation of the two layers may thenbe detected by determining the relative electrical resistance of thememory cell. If the magnetization orientation of its magnetic layers aresubstantially parallel, a memory cell is typically in a low resistancestate. In contrast, if the magnetization orientation of its magneticlayers is substantially anti-parallel, the memory cell is typically in ahigh resistance state. Thus, ideally, in typical magneto-resistivememories, binary logic states are stored as binary magnetizationorientations in magneto-resistive materials and are read as the binaryresistance states of the magneto-resistive memory cells containing themagneto-resistive materials.

Giant magneto-resistive (GMR) and tunneling magneto-resistive (TMR)memory cells are two common types of memory cells that take advantage ofthis resistance behavior. In GMR cells, the flow of electrons through aconductor situated between a free magnetic layer and a pinned magneticlayer is made to vary, depending on the relative magnetizationorientations of the magnetic layers on either side of the conductor. Byswitching the magnetization orientation of the free magnetic layer, theelectron flow through the conductor is altered and the effectiveresistance of the conductor is changed.

In TMR cells, an electrical barrier layer, rather than a conductor, issituated between a free magnetic layer and a pinned magnetic layer.Electrical charges quantum mechanically tunnel through the barrierlayer. Due to the spin dependent nature of the tunneling, the extent ofelectrical charges passing through the barrier vary with the relativemagnetization orientations of the two magnetic layers on either side ofthe barrier. Thus, the measured resistance of the TMR cell may beswitched by switching the magnetization orientation of the free magneticlayer.

Switching the relative magnetization orientations of themagneto-resistive materials in the memory cell by applying an externalmagnetic field is the method commonly used to write a logic state to amagneto-resistive memory cell. The magnitude of the applied magneticfield is typically sufficient to switch the magnetization orientation ofthe free magnetic layer, but not large enough to switch the pinnedmagnetic layer. Thus, depending on the desired logic state, anappropriately aligned magnetic field is applied to switch themagnetization orientation of the free magnetic layer so that themagneto-resistive memory cell is in either a high or a low resistancestate.

Magneto-resistive memory cells are typically part of an array of similarcells and the selection of a particular cell for writing is usuallyfacilitated by use of a grid of conductors. Thus, magneto-resistivememory cells are usually located at the intersections of at least twoconductors. A magneto-resistive memory cell is typically selected forwriting by applying electrical currents to two conductors that intersectat the selected magneto-resistive memory cell. With current flowingthrough it, each conductor generates a magnetic field and, typically,only the selected magneto-resistive memory cell receives two magneticfields, one from each conductor. The current flowing through eachconductor is such that the net magnetic field from the combination ofboth these magnetic fields is sufficient to switch the magnetizationorientation of the selected cells. Other magneto-resistive memory cellsin contact with a particular conductor usually receive only the magneticfield generated by that particular conductor. Thus, the magnitudes ofthe magnetic fields generated by each line are usually chosen to be highenough so that the combination of both fields switches the logic stateof a selected magneto-resistive memory cell but low enough so that theother magneto-resistive memory cells which are subject to only onemagnetic field do not switch.

In addition to the two conductors for writing, memory arrays with threeconductors connecting magneto-resistive memory cells have also beendeveloped. The additional conductor may be used exclusively for sensingthe resistance of a particular memory cell, allowing another conductorto be used exclusively for writing. In this way, writing and readingoperations may be performed simultaneously, increasing the speed of dataaccess.

Magneto-resistive memory technology continues to mature and workcontinues in refining implementation of magneto-resistive memory cells.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention providemagneto-resistive memory cell structures which minimize disruptions tothe magnetization orientation of the free magnetic layer caused bypinned magnetic layer magnetic fields. In one embodiment, amagneto-resistive memory cell has an additional pinned magnetic layerand a pinned magnetic layer, with the free magnetic layer and theadditional pinned magnetic layer on either side of the pinned magneticlayer. The additional magnetic layer has a magnetization orientationsubstantially anti-parallel to the magnetization orientation of thepinned magnetic layer. The resulting magnitude of the net magnetic fieldfrom the pinned magnetic layer and the additional pinned magnetic layeris too small to affect the magnetization orientation of the freemagnetic layer or the magnetization orientation of the free magneticlayers of neighboring magneto-resistive memory cells.

Also provided are methods for constructing the magneto-resistive memorycells of the present invention. In one embodiment, ferromagneticmaterials and the thicknesses of the magnetic layers made from thematerials are first selected such that the magnitude of the magneticfield from one layer is substantially equal to the magnitude of themagnetic field from the other layer. The magnetic layers are thenformed. After formation, the magnetization orientation of each layer isfixed in an opposite direction from the other layer.

Other features and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theteachings of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized representation of the magneto-resistive behaviorof a prior art magneto-resistive memory cell;

FIG. 2 is an additional generalized representation of themagneto-resistive behavior of a prior art magneto-resistive memory cell;

FIG. 3 is a representation of the magneto-resistive behavior of amagneto-resistive memory cell of the present invention;

FIG. 4 is a cross-sectional view of one illustrative embodiment of thepresent invention;

FIG. 5 is a cross-sectional view of another illustrative embodiment ofthe present invention; and

FIG. 6 is a top view of a magneto-resistive memory which incorporatesthe present teachings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To reliably store a state using magneto-resistive memory, it isdesirable to have predictability in switching behavior. As such, it isdesirable to have a magneto-resistive memory cell that will switch to ahigh resistance state by application of a magnetic field of apredictable magnitude and that will also switch to a low resistancestate by application of a magnetic field of a predictable magnitude inthe opposite direction. Moreover, predictability in switching behaviorassociated with a particular applied magnetic field will improve theability to select a particular magneto-resistive memory cell in amagneto-resistive memory cell array. Unfortunately, interactions betweenthe magnetic fields of the pinned and free magnetic layers commonlyfound in prior magneto-resistive memory cells contribute tounpredictability in switching and, thus, undermine the ability toselectively switch cells.

At short distances on the order of less than about 20 Å, theinteractions of the magnetic fields of two ferromagnetic layers mayinclude either antiferromagnetic or ferromagnetic coupling.Antiferromagnetic coupling tends to cause the magnetization orientationsof the two layers to be parallel, while ferromagnetic coupling tends tocause anti-parallel magnetization orientations. The extent of each typeof coupling oscillates depending, inter alia, on the distance betweenthe two layers.

Thus, where antiferromagnetic coupling dominates, an applied magneticfield must overcome the influence of this coupling to switch a freemagnetic layer to the parallel orientation, while the sameantiferromagnetic coupling field will augment an magnetic field appliedto switch the same free magnetic layer to the anti-parallel orientation.Thus, as represented in FIG. 1, the magnitude of the applied magneticfield needed to switch the free magnetic layer to the low resistancestate is increased while the magnitude of the applied magnetic fieldneeded to switch the free magnetic layer to the high resistance state isdecreased. As a consequence, antiferromagnetic coupling fields typicallyincrease the current needed to write the magneto-resistive memory cellto the low resistance state and may cause accidental writing to the highresistance state. In extreme cases, the applied magnetic fields may beinsufficient to overcome the demagnetization fields and the freemagnetic layer may remain in the anti-parallel orientation.

Alternatively, where ferromagnetic coupling dominates, an appliedmagnetic field must overcome the influence of this coupling to switch afree magnetic layer to the anti-parallel orientation, while these samecoupling fields will augment a magnetic field applied to switch a freemagnetic layer to the parallel orientation. Thus, as represented in FIG.2, ferromagnetic coupling typically increases the power needed to writethe magneto-resistive memory cell to the high resistance state and maycause accidental writing to the low resistance state. In extreme cases,ferromagnetic coupling fields may cause a magneto-resistive memory cellto remain in the low resistance state regardless of the applied magneticfield.

In less extreme cases, where antiferromagnetic coupling andferromagnetic coupling is strong, but where an applied magnetic field isable to switch the magnetization orientation of the free magnetic layer,the coupling between the free and fixed magnetic layers may still besufficient to cause the magnetization orientation of the free layer toswitch back to its previous orientation. In such cases, depending on thestrength of coupling with the fixed magnetic layer, the orientation ofthe free layer may switch spontaneously, undermining the ability of thecell to be used as a non-volatile memory device. Similarly, less extremeantiferromagnetic coupling and ferromagnetic coupling between the pinnedmagnetic layer of a magneto-resistive memory cell and the free magneticlayers of neighboring magnetoresistive memory cells may causedemagnetization of those neighboring memory cells.

In addition, manufacturing non-uniformity in the thickness ofnonmagnetic layers separating two ferromagnetic layers, in conjunctionwith the distance related oscillations in the degree ofantiferromagnetic and ferromagnetic coupling, contribute tounpredictability in free magnetic layer switching behavior. Further, therelative influence of antiferromagnetic and ferromagnetic couplingfields on free magnetic layer magnetization may vary among themagneto-resistive memory cells in a magneto-resistive memory array andmay vary between different magneto-resistive memory arrays due tovariations in the patterning steps and/or deposition steps of devicemanufacture. Such variations may lead to unpredictable switchingbehavior and further undermine memory cell selectivity during writeoperations.

At longer distances, where the distances between magnetic layers aregreater than about 20 Å, magnetostatic interactions may dominate. Inthis case, stray magnetic fields originating from the pinned layer mayinteract with and alter the magnetization orientations of the freelayer. Moreover, these stray magnetic fields may interact with themagnetization orientations of free magnetic layers in neighboringmagneto-resistive memory cells, further undermining the switchingpredictability of those memory cells. Magnetostatic interactions are ofparticular concern as work continues on decreasing the distance betweenmemory cells to increase the memory cell density of magneto-resistivememory cell arrays.

U.S. Pat. No. 6,172,904 discloses one possible structure for addressingthe problem of unpredictable switching behavior caused by pinnedmagnetic layer and free magnetic layer coupling within a particularmemory cell. That patent describes a structure, illustrated in FIG. 4 ofthat patent, with a free magnetic layer 4 between two pinned magneticlayers, 6 and 34, with anti-parallel magnetization orientations M1 andM4. The pinned layers 6 and 34 couple to the free magnetic layer 4.Thus, coupling to one pinned layer 6 forces the magnetizationorientation M3 of the free magnetic layer 4 in one direction. However,because the structure contains another pinned layer 34 with an oppositemagnetization orientation M4 and a magnetic field of the same magnitude,the second pinned layer 34 also couples to the free magnetic layer 4.Each pinned magnetic layer 6 and 34 affects the free magnetic layer 4 inan equal and opposite way, resulting in no net tendency to push themagnetization orientation M3 of the free magnetic layer 4 in anyparticular direction.

The preferred embodiments of the present invention takes advantage ofthe interactions between two fixed layers to increase predictability inswitching behavior. As represented in FIG. 3, and discussed furtherbelow, the interactions between the two fixed layers allow for switchingbehavior in which the magnitude of the applied magnetic fields used toswitch a magneto-resistive memory cell from a low resistance state to ahigh resistance state and vice versa are substantially equal. Because ofthis magneto-resistive behavior, the magnitudes of the current used towrite a logic “0” or “1” are substantially equal, and switchingpredictability and control is increased. In addition, the couplingbetween the free and pinned magnetic layers of neighboring memory cellsis reduced.

FIG. 4 is a cross-sectional view of one illustrative embodiment of theinvention, implemented in the context of a giant magnetoresistive (GMR)memory array. A magneto-resistive memory cell 2 comprises a free layer4, a pinned magnetic layer 6, and an additional pinned magnetic layer 8.The pinned magnetic layer 6 has a fixed magnetization orientation M1,the additional pinned magnetic layer 8 has a fixed magnetizationorientation M2, and the free magnetic layer 4 has a switchablemagnetization orientation M3. While the magnetic orientations M1 and M2are shown pointing in particular directions, in another embodiment thesedirections may be reversed, so long as their magnetization orientationsremain anti-parallel.

To ensure that a net magnetic moment of the pinned magnetic layers 6 and8 will not affect the magnetization of the free magnetic layer, theorientations, thicknesses, and materials of the pinned magnetic layers 6and 8 are selected so that the magnitude of the magnetic field of thepinned magnetic layer 6 is substantially opposite to the magnitude ofthe magnetic field of the additional magnetic layer 8. Because amagnetic field associated with a material may vary depending on the typeand quantity of that material, the materials used and the thicknesses ofthese materials should be taken into account. For example, this may beaccomplished by using substantially similar magneto-resistive materialswith a substantially similar thickness for each pinned magnetic layer 6and 8 and orienting the pinned magnetic layers 6 and 8 in substantiallyopposite magnetization directions. Similarly, where the materialscomprising each pinned magnetic layer differ in the strength of themagnetic field associated with that material, the respective thicknessesof the pinned magnetic layers 6 and 8 may be selected so as tocompensate for the different materials properties. For example, wherethe magnetic field associated with the material comprising theadditional pinned magnetic layer 8 is of a greater magnitude than themagnetic field associated with the pinned magnetic layer 6, thethickness of pinned magnetic layer 6 may be increased to compensate. Assuch, the net magnetic field from the pinned magnetic layers 6 and 8 istoo small to affect the magnetization orientation of the free magneticlayer 4 or a similar free magnetic layer (not shown) in a neighboringmagneto-resistive memory cell, in which case an applied magnetic fieldof a particular magnitude can switch a magneto-resistive cell 2 to thecell's high resistance state and an opposite applied magnetic field ofapproximately the same magnitude can switch the same magneto-resistivecell 2 to the cell's low resistance state.

A separating layer 10 preferably separates the pinned magnetic layer 6from the additional pinned magnetic layer 8. The separating layer 10 ispreferably comprised of a conductive material that enhances coupling ofthe magneto-resistive materials to each other. In this capacity,ruthenium or a similar material may be used. The thickness of theruthenium layer should be in a range to allow antiferromagnetic couplingbetween the pinned magnetic layer 6 and the additional pinned magneticlayer 8. Preferably the ruthenium layer is 7-9 Å thick to allow for theantiferromagnetic coupling.

In addition, the pinned magnetic layers 6 and 8 are preferably comprisedof ferromagnetic materials, including cobalt, iron-cobalt, nickel iron,nickel-iron-cobalt, or similar material. Where such soft ferromagneticmaterials are used, the magnetization orientation of the additionalpinned magnetic layer 8 is preferably fixed by an adjacent layer 14 ofantiferromagnetic material, in contact with and located directly belowthe additional pinned magnetic layer 8. The antiferromagnetic materialmay be iron-manganese, nickel-manganese, iridium-manganese,platinum-manganese, or similar material. The skilled artisan willappreciate that the anti-parallel configuration of pinned magnetic layer6 and additional pinned magnetic layer 8 allow for use ofantiferromagnetic materials of reduced pinning strength, in comparisonto magneto-resistive memory cells with a single pinned layer. In anotherembodiment, the magnetization orientation of the additional pinnedmagnetic layer 8 is fixed by a layer 14 comprising a permanent magnetmaterial.

In addition, where the coercivities of the pinned magnetic layer 6 andthe additional pinned magnetic layer 8 are substantially similar, theadjacent layer 14 may also be useful in the manufacture of themagneto-resistive memory 2. By fixing the magnetization orientation ofthe additional pinned magnetic layer 8 during the manufacturing process,the pinned magnetic layer 6 may be magnetically oriented, withoutsimultaneously changing the magnetization orientation of the additionalpinned magnetic layer 8. The skilled artisan will recognize, however,that an adjacent layer 14 is not necessary to the manufacturing process.For example, simultaneous changing of the magnetization orientation ofthe additional pinned magnetic layer 8 is a reduced concern where thecoercivities of the pinned magnetic layer 6 and the additional pinnedmagnetic layer 8 are not similar, e.g. where the additional pinnedmagnetic layer 8 is formed of a material of higher coercivity than thepinned magnetic layer 6 or where the additional pinned magnetic layer 8is formed of a permanent magnet material.

As with the pinned magnetic layers 6 and 8, the free magnetic layer 4 ispreferably also comprised of a soft ferromagnetic material such ascobalt, iron-cobalt, nickel iron, or nickel-iron-cobalt.

The free magnetic layer 4 is separated from the pinned magnetic layer 6by a non-magnetic interlayer 12. In a preferred embodiment, thenon-magnetic interlayer 12 comprises a conductor such as copper to takeadvantage of the giant magneto-resistive effect, where a read currentflows through the conductor of non-magnetic interlayer 12.

FIG. 5 is a cross-sectional view of another illustrative embodiment ofthe present invention wherein like features are referenced by likenumerals. In this embodiment, the additional pinned magnetic layer 8 iscomprised of a permanent magnet, the orientation of which may be fixedby exposure to a large external magnetic field. As such, themagneto-resistive memory cell 2 does not require an antiferromagneticlayer 14 to fix the orientation of additional pinned magnetic layer 8.In another embodiment, the additional pinned magnetic layer 8 comprisesa ferromagnetic material with high coercivity such that, in the presenceof applied magnetic fields of magnitudes in a range sufficient to switchthe free magnetic layer, the magnetic moment of this layer isessentially fixed by its intrinsic magnetic anisotropy.

FIG. 6 is a top view of an illustrative magneto-resistive memory 30 thatincorporates the present teachings. The magneto-resistive memory 30includes an array of magneto-resistive memory cells, including themagneto-resistive memory cell 2 and additional magneto-resistive memorycells 24-28, each similarly made. Each magneto-resistive memory cell islocated at the intersection of at least two conductors, one each fromthe sets of conductors, 16, 18 and 20, 22.

The sets of conductors allow reading from and writing to themagneto-resistive memory cells. In one embodiment, conductors 16 and 18are perpendicular to conductors 20 and 22. In other embodiments, theangle 32 formed by conductors 16 and 18 with conductors 20 and 22 mayvary so long as a magnetic-resistive memory cell 2 at the intersectionof two conductors may be selected and switched by a current flowingthrough the conductors.

While shown forming a rectangular shape, the present teachings do notdepend on and, so, do not limit the shape of the magneto-resistivememory cells 2 and 24-28. Further, relative sizes of the conductor lines16-22 and the magneto-resistive memory cells 2 and 24-28 areillustrative only. Here also, the present teachings do not depend onand, so, do not limit the relative sizes of the conductor lines 16-22and the magneto-resistive memory cells 2 and 24-28. Further, whilemagneto-resistive memory cells are typically connected to twoconductors, and are illustrated as such, it will be appreciated thatthis figure only illustrates that, at a minimum, two conductors arenecessary for writing to the memory cell. Additional conductors, e.g. anadditional, separate conductor (sense line) to sense the resistance ofthe memory cell, are not illustrated but can be added. In addition,while the order of each layer relative to other layers should bemaintained, the present teachings do not limit the orientation of thestack of layers as a whole. For example, the stacks illustrated in FIGS.4 and 5 may be constructed on a substrate such that they appear upsidedown relative to the figures.

Consequently, although this invention has been described in terms of acertain preferred embodiment and suggested possible modificationsthereto, other embodiments and modifications that may suggest themselvesand be apparent to those of ordinary skill in the art are also withinthe spirit and scope of this invention. Accordingly, the scope of thisinvention is intended to be defined by the claims that follow.

1. An integrated circuit comprising a magneto-resistive memory cell, themagneto-resistive memory cell comprising: a free magnetic layer; anon-magnetic interlayer, wherein the non-magnetic interlayer comprises aconductor and is in contact with the free magnetic layer; a pinnedmagnetic layer, wherein the pinned magnetic layer is in contact with thenon-magnetic interlayer; and an additional pinned magnetic layer,wherein the pinned magnetic layer is between the free magnetic layer andthe additional pinned magnetic layer and wherein a magnetizationorientation of the pinned magnetic layer is substantially anti-parallelto a magnetization orientation of the additional pinned magnetic layer,wherein a magneto-resistive material comprising the pinned magneticlayer is different from a magneto-resistive material comprising theadditional pinned magnetic layer.
 2. The integrated circuit of claim 1,wherein the pinned magnetic layer and additional pinned magnetic layerhave preselected thicknesses such that a magnitude of a magnetic fieldof the pinned magnetic layer is substantially equal and substantiallyopposite to a magnitude of an additional magnetic field of theadditional pinned magnetic layer.
 3. The integrated circuit of claim 1,wherein a first magnitude of an applied magnetic field for switching themagnetization orientation of the free magnetic layer in a firstdirection is about 75-125 percent of a second magnitude of an appliedmagnetic field for switching the magnetization orientation of the freemagnetic layer in a direction substantially opposite to the firstdirection.
 4. The integrated circuit of claim 1, wherein the additionalpinned magnetic layer comprises a ferromagnetic material withmagnetization orientation pinned by an adjacent layer.
 5. The integratedcircuit of claim 4, wherein the adjacent layer comprises anantiferromagnetic material.
 6. The integrated circuit of claim 4,wherein the adjacent layer comprises a permanent magnet material.
 7. Theintegrated circuit of claim 1, wherein the pinned magnetic layercomprises a permanent magnet.
 8. The integrated circuit of claim 1,wherein the additional pinned magnetic layer comprises a ferromagneticmaterial with coercivity sufficiently high such that its magnetizationorientation remains fixed in the presence of an applied magnetic fieldof a magnitude sufficient to switch the magnetization orientation of thefree magnetic layer.
 9. The integrated circuit of claim 1, wherein thepinned magnetic layer and the additional pinned magnetic layer areseparated by a separating layer.
 10. The integrated circuit of claim 9,wherein the separating layer is ruthenium.
 11. The integrated circuit ofclaim 1, wherein the nonmagnetic interlayer comprises copper.
 12. Theintegrated circuit of claim 11, wherein the magneto-resistive memorycell is formed within a giant magneto-resistive (GMR) memory array. 13.A method of constructing a magneto-resistive memory cell in anintegrated circuit, comprising: forming a first magnetic layer; forminga non-magnetic interlayer, wherein the non-magnetic interlayer comprisesa conductor; forming a second magnetic layer without forming anothermagnetic layer between the first magnetic layer and the second magneticlayer; forming a first fixed magnetic layer by applying a first magneticfield to fix a magnetization orientation of the first magnetic layer;and forming a second fixed magnetic layer by applying a second magneticfield to fix a magnetization orientation of the second magnetic layer inan opposite direction from the magnetization orientation of the firstmagnetic layer, wherein a magnetic material used in forming the firstfixed magnetic layer is different from a magnetic material used informing the second fixed magnetic layer.
 14. The method of claim 13,wherein a set of ferromagnetic and antiferromagnetic coupling fields ofthe second fixed magnetic layer balance an additional set offerromagnetic and antiferromagnetic coupling fields from the first fixedmagnetic layer.
 15. The method of claim 13, wherein the first magneticlayer and the second magnetic layer have substantially the samethickness.
 16. The method of claim 13, wherein the first magnetic layerand the second magnetic layer are formed sequentially.
 17. The method ofclaim 16, wherein the conductor comprises copper.
 18. Amagneto-resistive memory cell, comprising: a free magnetic layer; apinned magnetic layer; a non-magnetic interlayer between the freemagnetic layer and the pinned magnetic layer, wherein the non-magneticinterlayer comprises a conductor; and an additional pinned magneticlayer, wherein the pinned magnetic layer is between the free magneticlayer and the additional pinned magnetic layer and wherein amagnetization orientation of the pinned magnetic layer is substantiallyanti-parallel to a magnetization orientation of the additional pinnedmagnetic layer, wherein a magneto-resistive material comprising thepinned magnetic layer is different from a magneto-resistive materialcomprising the additional pinned magnetic layer.
 19. Themagneto-resistive memory cell of claim 18, wherein the pinned magneticlayer and additional pinned magnetic layer have preselected thicknessessuch that a magnitude of a magnetic field of the pinned magnetic layeris substantially equal and substantially opposite to a magnitude of anadditional magnetic field of the additional pinned magnetic layer. 20.The magneto-resistive memory cell of claim 18, wherein a first minimummagnitude of an applied magnetic field for switching a magnetizationorientation of the free magnetic layer in a first direction is about75-125 percent of a second minimum magnitude of an applied magneticfield for switching the magnetization orientation of the free magneticlayer in a direction substantially opposite to the first direction. 21.A memory device comprising a magneto-resistive memory cell, the memorycell comprising: a free magnetic layer; a non-magnetic interlayer,wherein the non-magnetic interlayer comprises a conductor and is incontact with the free magnetic layer; a pinned magnetic layer, whereinthe pinned magnetic layer is in contact with the non-magneticinterlayer; and an additional pinned magnetic layer, wherein the pinnedmagnetic layer is between the free magnetic layer and the additionalpinned magnetic layer and wherein a magnetization orientation of thepinned magnetic layer is substantially anti-parallel to a magnetizationorientation of the additional pinned magnetic layer, wherein a firstmagnitude of an applied magnetic field for switching the magnetizationorientation of the free magnetic layer in a first direction is about75-125 percent of a second magnitude of an applied magnetic field forswitching the magnetization orientation of the free magnetic layer in adirection substantially opposite to the first direction, wherein amagneto-resistive material comprising the pinned magnetic layer isdifferent from a magneto-resistive material comprising the additionalpinned magnetic layer.
 22. The memory device of claim 21, wherein aseparating layer separates the pinned and additional pinned magneticlayers.
 23. The memory device of claim 22, wherein the separating layercomprises ruthenium.
 24. The memory device of claim 23, wherein theseparating layer is 7-9 Å thick.
 25. The memory device of claim 21,wherein the magneto-resistive memory cell is located at an intersectionof at least two conductors.
 26. The memory device of claim 21, whereinthe pinned and the additional pinned magnetic layers are formed of amaterial chosen from the group consisting of cobalt, iron-cobalt, nickeliron and nickel-iron-cobalt.
 27. The memory device of claim 26, whereina magnetic orientation of the additional pinned magnetic layer is fixedby a layer of antiferromagnetic material.
 28. The memory device of claim27, wherein the antiferromagnetic material is chosen from the groupconsisting of iron-manganese, nickel-manganese, iridium-manganese andplatinum-manganese.
 29. The memory device of claim 26, wherein the freemagnetic layer is formed of a material chosen from the group consistingof cobalt, iron-cobalt, nickel iron and nickel-iron-cobalt.
 30. Thememory device of claim 21, wherein the non-magnetic interlayer comprisescopper.